This invention is applicable to refining various types of coal; anthracite, bituminous, and lignite. Its primary application will be with coals burned for industrial purposes. Depending upon the source, these coals contain various contaminants that may produce environmental pollutants in the combustion gas or the ash residue. Various methods of washing, mechanical separation and chemical reaction have been and are being used to reduce these contaminants before the coal is burned.
Sulfur is a significant contaminant of particular concern for industrial coal burning plants. Coals containing a high sulfur content can release a significant amount of sulfur oxides in combustion gases. The most common form of sulfur oxide in combustion gas is sulfur dioxide (SO2), and it is of particular environmental concern. Sulfur dioxide reacts with oxygen, usually in the pre sence of a catalyst such as nitrogen dioxide (NO2), to form sulfur trioxide (SO3), which then reacts with water molecules in the atmosphere to form sulfuric acid (H2SO4) that is returned to the Earth as acid rain. Consequently, environmental concerns about these pollutants in coal combustion gas have produced government regulations limiting the emissions of sulfur oxides (SOx) and nitrogen oxides (NOx). Nitrogen oxide emissions from coal combustion can be reduced by burner technologies, such as fluidized bed combustion. For sulfur oxide reduction, there are flue gas desulfurization systems for scrubbing the sulfur oxides from coal combustion gases in the flue stacks of modern coal-fired electrical generation plants, but it is generally more effective to reduce the sulfur content of any high sulfur coal prior to its combustion.
Chemical analyses of coal generally report the sulfur content in three categories, sulfate sulfur, pyritic sulfur and organic sulfur, which combine to make the total sulfur content of a coal sample. Most analysis protocols measure pyritic sulfur and organic sulfur, along with total sulfur content. The difference between the pyritic and organic contribution and the total sulfur is then attributed to sulfates. The type of sulfate may be a calcium sulfate, such as gypsum, or ferrous sulfates produced by weathering of exposed coal. Regardless of type, separating sulfates from coal is relatively easy, since sulfates can be can be dissolved in diluted acid solutions or other solvents.
Pyritic sulfate is primarily iron disulfide (FeS2), a crystalline mineral known as pyrite. Pyrite frequently occurs in veins and beds near to or interwoven through coal seams. Pyrite is not soluble in water or weak acid solution. However, pyritic sulfates have a specific gravity 3 to 4 times greater than the coal. Thus, much of the pyritic form of sulfur can be separated from coal by traditional methods of gravity concentration, such as the dense medium separators or centrifuges commonly used in coal washing.
Organic sulfur is part of the coal itself, linked by chemical bonds. Organic sulfur has traditionally been difficult to remove because it cannot be separated from the coal without breaking the chemical bond. Oxidation reactions can be used to break the bonds and free the sulfur in other forms for removal from the coal matrix.
Consequently, in view of these different forms of sulfur content, the prior art of coal refining for sulfur reduction includes a wide range of processes, from simple washing in a solvent solution or washing in combination with dense media separation and/or froth flotation to dissolve most of the sulfate and separate much of the pyritic sulfur from the coal, to the use of chemical oxidants, oxidative enzymes and microbial desulfurization methods.
Chemical reagents have also been suggested for more aggressive reduction of pyritic sulfur. For example, the Meyers Process described in the article Chemical Removal of Prytic Sulfur from Coal, and in U.S. Pat. Nos. 3,926,575 and 3,917,465 (Meyers) is directed to the removal of pyritic sulfur by chemical reaction using ferric chloride or ferric sulfate as an oxidizing agent. It acknowledges that pyrite is insoluble in water, and that the acids commonly used to dissolve most inorganic salts (and sulfates) will not dissolve pyrite. Therefore, an oxidizing agent is used in the Meyers Process to convert the pyrite to sulfates or elemental sulfur, which are soluble in a diluted acid solution. The Meyers Process is based upon the postulate that ferric chloride and ferric sulfate are more selective to pyrite oxidation than to coal oxidation, with ferric sulfate being the preferred agent. Using reaction temperatures of about 100° C., Meyers reports from 40 to 70% removal of pyritic sulfur from bituminous coal by using ferric sulfate or ferric chloride as oxidation agents, followed by a neutralization wash in toluene.
There have also been chemical processes to reduce organic sulfur along with the pyrite. A process of coal desulphurization described by Hsu, et al in U.S. Pat. No. 4,081,250 uses a chlorine gas bubbled through a slurry of moist coal in a chlorinated solvent to wash away pyritic sulfur and to convert organic sulfur into soluble sulfates. The chlorinated coal is then separated, hydrolyzed and de-chlorinated by heating at 500° C.
Other processes eliminate a need for external heat by inducing an exothermic oxidation reaction in the coal over a brief period. U.S. Pat. No. 4,328,002 (Bender) describes a process of this type in which the coal is pretreated with a dilute aqueous suspension of an oxidizing agent, washed with water, and then sprayed with or immersed in a concentrated solution of the oxidizing agent for 1 to 2 minutes, during which time the exothermal reaction peeks. A later patent to Bender, U.S. Pat. No. 4,560,390, describes, however, that the exposure time to the oxidizing agent solution can be reduced to as short as 22-30 seconds exposure time when the reaction takes place inside of a hydrocyclone or a dense media classifier.
In view of these varied prior methods of treatment, an object of this invention is to find an effective and cost efficient coal refining process that can be practiced on an industrial scale to substantially reduce total sulfur content, including organic sulfur, from coal. The concurrent reduction of other coal contaminants and the increase in BTU output in the processed are welcome additional effects.